“Antonette Pepe is a fireball, and she’s burning up about something,” John O’Brien said by way of introducing the kick-ass School Committee member on this morning’s Bax & O’Brien show.

I’ll say: Pepe appeared on the show this morning to expose what she calls Mayor Domenic Sarno’s attempt to “blackmail” her. According to Pepe, earlier this fall she approached Sarno (who, as mayor, chairs the School Committee) to ask for his support of her bid to be the body’s vice chair in 2010. He agreed, she said.

But by Election night, when Pepe checked back in to make sure the mayor was still on board, he was singing a different tune, she said. According to Pepe, Sarno told her, “You will only get my vote for vice chair as long as you support [School Superintendent] Dr. Ingram at all costs.”

In addition, Pepe told Bax & O’Brien, Sarno also told her, “You have to keep your mouth shut about the federal building”—a reference to Pepe’s public questioning of a plan to move the School Department into the former federal courthouse on Main Street. Pepe, as well as City Councilor Tim Rooke and others, say it’s a bad plan that will cost taxpayers too much. They also question why the city didn’t seek competitive bids for a new School Department lease, in the hope of finding a better deal.

When pressed by the hosts about whether the mayor used the exact words “keep your mouth shut,” Pepe said the phrase he used might have been “keep quiet”—but in either case, the message was clear: Sarno was trying to buy her silence with the offer of political support.

If, indeed, Sarno did that, it was a grave miscalculation on his part—as he, and anyone else knows Pepe, should have been able to guess. From her days as the president of the school paraprofessional units, Pepe has earned a well-deserved reputation as an outspoken, fearless leader who bows down to no one—and who isn’t afraid to expose the dark side of city politics.

In an article posted on MassLive this morning, Sarno denied Pepe’s story, calling her accusation that he tried to blackmail her “out of line, an outrageous statement.”

Adding insult to injury, Pepe had supported Sarno in this fall’s campaign against his challenger, City Councilor Bud Williams. I can recall conversations I’ve had with Pepe in the past in which she credited Sarno with being a man who sticks to his word. Now, she told Bax & O’Brien, “He should look at himself in the mirror and be embarrassed, be ashamed.”

Pepe also charged that Sarno persuaded two new incoming School Committee members, Denise Hurst and Norman Roldan, to drop their previously pledged support for the vice chairmanship—a situation, she says, that calls their leadership abilities into question. “How can you trust people who can be so easily swayed by the mayor?” she asked.

Pepe’s accusations, meanwhile, call to mind similar accusations by the owners of the SkyPlex nightclub, who say the mayor denied them permits to hold under-21 nights at the club as retaliation for their failure to host a fundraiser for him. The owners have sued the city over the permit denials. In court this summer, Sarno testified that that accusation was untrue.